Method and apparatus for reducing brightness flickering of television images

ABSTRACT

Method of reducing the flickering of the brightness of the video image from a television camera with a semiconductor sensor directed at the beam path of a motion picture camera with a rotating mirror shutter, in which the charge of a video half-image is integrated and the signal voltage of the video half-images serves as a video input signal to a screen and/or a video recorder. The video output signal of each video half-image is multiplied by a correction factor which is a function of the ratio of the average exposure time of a video half-image to the actual exposure time of the video half-image which depends on the rotational speed and phase of the mirror shutter as well as the integration window of the video image.

The invention relates to a method for reducing the flickering of thebrightness of a television image from a television camera aimed at thebeam path of a television camera as well as a device for working themethod.

BACKGROUND OF THE INVENTION

It is known that a television camera can be disposed in the beam path ofa motion picture camera making it possible to view on a screen or recordon a video recorder the frame independently of the optical findermounted on the motion picture camera. This also makes it possible toview the frame at a point remote from the motion picture camera and toevaluate the frame immediately without time-consuming development of themotion picture film. The television camera is located in the vicinity ofthe optical finder of the motion picture camera, i.e. in the beam pathbehind the rotating mirror shutter, with a beam splitter sending onepart of the light reflected by the rotating mirror shutter into theoptical finder and another part to the lens of the television camera.

Since the image received by the television camera depends on the takingspeed of the motion picture camera and hence on the rotational speed ofthe rotating shutter of the camera, at certain taking speeds a highlydisturbing flickering of the brightness in the video image occurs.Therefore when a television camera is adapted to a motion picture camerait is necessary when evaluating the video image generated by thetelevision camera to eliminate the highly disturbing flickering in thevideo image that results from using the light reflected from the motionpicture camera mirror shutter with a mirror sector which is generally180°.

Without suitable additional electronics, when using a tube camera as atelevision camera and a rotational frequency of the mirror shutter aswell as a video image frequency of 25 Hz for example, a flickeringbrightness would result in the video image with a node and antinode inthe vertical. At different rotational frequencies of the mirror shutter,on the other hand, the antinode and node, depending on the differencebetween the rotational frequency of the mirror shutter and the videoimage frequency, move upward or downward constantly through the videoimage. Image modulation increases sharply as the rotational speed of themirror shutter decreases.

FIG. 1 is a block diagram explaining the principle of a televisionrecording from a tube camera 1 coupled with a motion picture camera.Motion picture camera 2 has a lens 21, a rotating mirror shutter 22, anda film plane 25 in which the motion picture film is moved. The lightreflected from rotating mirror shutter 22 passes through a ground glass24 and a beam splitter 26, from which a portion of the beams isconducted to optical finder 27 of motion picture camera 2 and a portionthrough a camera connection 28 to television camera 1.

The output of television camera 1 is connected by an analog-digitalconverter 3 and an electronic switch 40 with two video field memories41, 42, whose output is likewise connected through an electronic switch43 with a digital-analog converter 6. Two video fields make up one videoimage.

A sensor 23 provided in motion picture camera 2 detects the rotationalspeed and position or phase of rotating mirror shutter 22 and delivers acorresponding signal to a control electronic circuit 5, which controlsboth television camera 1 and video field memories 41, 42 as well aselectronic switches 40, 43 and digital-analog converter 6. The output ofdigital-analog converter 6 is normally connected to a screen 7 and/or avideo recorder 8.

The coupling of motion picture camera 2, television camera 1, and thetwo video field memories 41, 42 makes it possible to eliminate theflickering in the brightness of a tube camera by controlling thescanning of the taking tube of television camera 1 by the position androtational speed signal of mirror shutter 22 output in sensor 23. Witheach revolution of mirror shutter 22, exactly one video video field isscanned and recorded in one of the two video field memories 41 and 42.This video field memory 41 or 42 is then repeatedly read until a newvideo field is stored in the other field memory 42 or 41.

In contrast to television cameras with image tubes, television cameraswith semiconductor sensors have the advantage that the television cameraneed not be re-equipped for the special application of coupling with amotion picture camera or developed for such operation. In addition,television cameras with semiconductor sensors are operatedasynchronously and may be synchronized with other television signals aswell.

Another important advantage consists in the fact that no image memory isrequired, with costly control electronics and no analog/digital ordigital/analog converter.

Finally, the detail resolution in the vertical of a television camerawith semiconductor sensors corresponds to the motion picture camera andthe television standard, while this is reduced by half in the verticalin a tube camera with a memory system, since the video fields to bestored are always identical.

Given the advantages of a television camera with semiconductor sensors,the only disadvantage is that in a tube camera with a memory system avideo image can be "frozen" and displayed mixed with the current signalwith the motion picture camera not running.

SUMMARY OF THE INVENTION

The goal of the present invention is to provide a method and a devicefor reducing the flickering of the brightness of the video image from atelevision camera with semiconductor sensors directed at the beam pathof a motion picture camera.

The solution according to the invention provides a considerablereduction of the flickering in brightness in black-and-white and colortelevision cameras with complete resolution, is suitable for alltelevision standards, and has the advantage over image tube televisioncameras of asynchronous camera operation, low circuit cost, smallvolume, and low power consumption, which is especially important forbattery operation of the motion picture and television camera. Inaddition, an optimal detail resolution in the vertical is ensured. Tocorrect or reduce the flickering in brightness, it is merely necessaryto acquire a mirror shutter position and speed signal as well as thevideo signal from the television camera.

One advantageous embodiment of the solution according to the inventionis characterized by the fact that the position and rotational speed ofthe mirror shutter as well as the integration window for adjusting thetrue exposure time for each video field are recorded and the quotient ofthe average and actual exposure times is calculated, and by the factthat a function table is used to obtain a multiplication factor by whichthe video output signal is multiplied.

One advantageous device for working the method is characterized by thefact that a sensor which detects the rotational speed and phase of therotating mirror shutter is provided in the motion picture camera, theoutput signal of said sensor being connected with one input of aprocessor, whose second input is connected with the output of asynchronization signal detection stage which receives the video outputsignal from the television camera, by the fact that the output of theprocessor delivers a correction factor or multiplication factor to amultiplying element, to which the video output signal from thetelevision camera is applied, and by the fact that the multiplierelement is connected on the output side with the television screenand/or the television recorder.

This design of the device according to the invention is characterized bylow circuit costs and is especially suitable for a working range of themotion picture camera with a film speed of more than twenty frames persecond for nearly all applications. This distinguishes the use of atelevision camera with semiconductor sensors from a tube camera with amemory system, in which, with a television standard of 625 lines and afrequency of 50 Hz, operation is functional only in a working range from0 to 25 frames per second.

The invention will now be described in greater detail with reference tothe embodiment shown in the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a video recording circuit with a tubecamera with memory system;

FIG. 2 is a block diagram of a video recording device with a televisioncamera with semiconductor sensors;

FIG. 3 is a graph showing possible exposure times as a function of therotational frequency of the mirror shutter of a motion picture camera;and

FIG. 4 is a graph of the various signals in FIG. 2.

DESCRIPTION OF PARTICULAR EMBODIMENT

FIG. 2 shows a motion picture camera 2 which, as shown in FIG. 1, has alens 21, a rotating mirror shutter 22, and a film plane 25, in which amotion picture film is moved. Depending on the position of rotatingmirror shutter 22, the light beams received by lens 21 strike the motionpicture film in film plane 25 or pass through a ground glass 24 to abeam splitter 26, from which one part of the beams is conducted tooptical finder 27 or passes through an additional terminal 28 to atelevision camera with semiconductor sensors 1. Rotating mirror shutter22 is usually a mirror shutter with a mirror sector of 180°, but alarger mirror sector can also be provided.

A sensor 23, preferably in the form of a light barrier, is provided inthe vicinity of rotating mirror shutter 22, said sensor detecting therotational speed of mirror shutter 22 as well as its position, i.e. itsphase. The output signal from sensor 23 is fed to one input of aprocessor 9.

The video output signal of the television camera with semiconductorsensor 1 is fed to the input of a synchronization signal detectiondevice 10 as well as a multiplier element 12. Synchronization signaldetecting device 10 filters the video synchronization signal out of thevideo output signal of the television camera with semiconductor sensor 1and feeds an integration window signal to processor 9.

In the embodiment shown in FIG. 2, which relates to a television systemwith 625 lines and a frequency of 50 Hz, the output of processor 9 isconnected with a table memory 11 to which it gives a correction factorKF which determines a multiplication factor MP from the table, which isthen applied to a second input of multiplier element 12. The product ofthe two signals applied to multiplier element 12 are eventually outputto a screen 7 and/or a video recorder 8 as a corrected video outputsignal.

The television camera with semiconductor sensors 1 is a camera with aninterline-CCD sensor or frame-transfer sensors with a field integrationmode, in which the optical signals that are received are converted by animage converter directly into electrical signals. It is possible to useboth a "frame integration mode" and a "field integration mode". Thedifferent operating modes merely limit the range of application, butbasically permit use of the method according to the invention forreducing the flickering of the brightness.

In the preferred use of a television camera with interline-CCD sensors,suited particularly for an image frequency between 24 and 30 frames persecond in the motion picture camera, which permits a frame integrationmode, the integration time for one image element corresponds to the timeof two video fields. Two video fields make up one video image. Thenintegration of each pixel in a video field occurs at exactly the sametime, since reading of the integrated charges of the interline-CCDsensor takes place in parallel and not in serial fashion as in an imagetube. As will be described in greater detail below, with a rotationalfrequency of mirror shutter 22 that is equal to the video imagefrequency, the video image is absolutely free of flicker. At arotational frequency of mirror shutter 22 of motion picture camera 2that is different from the video image frequency, according to FIG. 2the image content of each image element is essentiallyamplitude-modulated.

The integration time for each video field of the semiconductor sensor isthe time during which the charge of the individual pixels in thesemiconductor sensor is integrated, in other words the time within whichthe semiconductor sensor receives light.

To eliminate or reduce the flickering of the brightness of a videoimage, the rotational speed and position or phase of rotating mirrorshutter 22 and the integration window of each individual video field arerecorded and from this the exposure time of the semiconductor sensor, inother words the duration of the actual illumination of the semiconductorsensor, is determined.

The integration window determines the position and size of theintegration interval for each video field of the semiconductor sensor.While the size of the integration window is standard-specific orcamera-specific, the position of the integration window can be derivedfrom the synchronization signal of the video signal. Since thesynchronization signal is preset, the beginning of the integration timeof the semiconductor sensor can be derived relative to thesynchronization signal.

This is determined by the fact that the reading of a video field givesthe integration time, i.e. the position of the integration window, sincethe integration has occurred during a space of time which ends with thereading of a new video field. For this reason the position of theintegration window can be determined from the synchronization signal ofthe video signal, which is filtered out by means of synchronizationsignal detection device 10.

By measuring the exposure time which lies somewhere in a shaded area ofpossible exposure times in FIG. 3, a correction factor is calculated bymeans of processor 9 from the quotient of the average exposure time andthe actual, i.e. measured, exposure time, so that the exposure time ofthe semiconductor sensor is standardized on the average exposure time.The correction factor

    KF=average exposure time/actual exposure time

is output to table memory 11, which determines from it a multiplicationfactor MP, where

    MP=f(KF)

and then

    MP≈(KF).sup.γ

This function in a television camera is usually an exponential functionwith the exponent 0.4<γ<1, which comes from the so-called "gammacorrection" of a television camera. The exposure of a CCD sensor resultsin a charge proportional to the exposure which is intergrated andresults in a voltage level proportional to the charge. This voltage iscorrected with a nonlinear element within the scope of gamma correctionand gives the video voltage.

Table memory 11 is a curve memory for the television camera in question,which, depending on the input voltage in question, outputs a correctedvideo voltage value. The multiplication factor MP output by the tablememory, like the video signal, is applied to one input of multiplierelement 12 where the signal voltage of the respective video field ismultiplied with the corresponding multiplication factor, so thatamplitude flicker is corrected.

On the basis of the description of the exposure time per video fieldusing the mirror shutter frequency according to FIG. 3 and the graph ofvarious signals plotted in FIG. 2 according to FIG. 4, the function ofthe method according to the invention as well as the device according tothe invention will now be described in greater detail.

FIG. 3 shows the various exposure times of a semiconductor sensor intelevision camera 1 as a function of the rotational frequency of mirrorshutter 22 of motion picture camera 2 according to FIG. 2. Between thetwo boundary curves shown in FIG. 3 are the possible exposure times ofthe semiconductor sensor; the area is shown shaded. The possibleexposure times are located between the maximum exposure time for thesemiconductor sensor of 40 milliseconds when the semiconductor sensor isconstantly illuminated and the minimum exposure times of thesemiconductor sensor of zero milliseconds, when the semiconductor sensoris not illuminated.

With a rotational frequency of mirror shutter 22 of less than 12.5revolutions per second, the semiconductor sensor of television camera 1receives images or video fields that are completely, i.e. maximallyilluminated as well as images and video fields which are notilluminated.

At 12.5 revolutions per second, one image is illuminated very brieflyand one image is illuminated completely.

As the rotational speed of mirror shutter 22 is increased further, theminimum exposure time for a video image increases while the maximumexposure time decreases, since mirror shutter 22 is rotating so fastthat during the integration interval the light reflected by mirrorshutter 22 does not always manage to reach the semiconductor sensor.

At a rotational speed of 25 revolutions per second, exactly one videofield is integrated for each revolution of mirror shutter 22, in otherwords each video field receives the same amount of light so that at arotational frequency of mirror shutter 22 which is equal to the videoimage frequency the video image is absolutely flicker-free, in otherwords there are no difference in brightness. As the rotational frequencyof mirror shutter 22 increases further, differences again appear betweenthe individual video fields, which are detected by the semiconductorsensor until, at a rotational frequency of 50 revolutions per second, aflicker-free image is again obtained, and so forth.

FIG. 4 shows the curve of the individual pulses or signals at the pointsindicated in FIG. 2.

FIG. 4a shows the light flux in the finder system of motion picturecamera 2 in the area of the beam splitter as it is received bytelevision camera 1. The light flux is composed of individual imageilluminations i, i+1, i+2, etc., which appear at intervals in the formof blocks.

FIG. 4b shows the integration windows of the video fields, withsuccessive integration windows overlapping one another. FIG. 4c showsthe measured, i.e. actual illumination of the semiconductor sensor as afunction of the position of the integration window as well as therotational speed and position of the mirror shutter, clearly indicatingthat the individual exposure values are subject to considerablevariations.

FIG. 4d shows the video input signal at the input of processor 9, withblocks n, n+1, n+2, n+3, . . . resulting from the individual exposureshaving amplitudes that correspond to the actual exposure values.

FIG. 4e shows the multiplication factor MP which fluctuates around avalue of 1 and upon which the ratio of the average exposure time to theactual exposure time depends.

FIG. 4f shows the video output signal at the output of multiplierelement 12, which by multiplying multiplication factor MP with the videoinput signal, i.e. the output signal of television camera 1, produces asignal with constant amplitude. The invention is not limited in itsembodiments to the preferred embodiment given above. Rather, a number ofvariations are possible which make use of the solution shown even inembodiments which are of fundamentally different design. In particular,the embodiment is not limited to implementation using discrete logicalmodules, but can advantageously employ programmed logic, preferablyusing a microprocessor.

I claim:
 1. Apparatus for reducing the flickering of the brightness of avideo image from a television camera comprisinga motion picture camerawith a rotating mirror shutter, a television camera directed at the beampath of said motion picture camera, said television camera including asemiconductor sensor in which the charge of a video field is integratedand the signal voltage of said video field charge serving as a videooutput signal, a synchronization signal detecting stage for receivingsaid video output signal from said television camera, a sensor fordetecting the rotational speed and phase of said rotating mirrorshutter, a processor having a first input from said sensor and a secondinput connected to the output of said synchronization signal detectingstage, said processor including means for generating a correction factorsignal, a multiplier, circuitry for applying said video output signaland said correction factor signal to said multiplier for generating amodified video output signal, and circuitry for applying said modifiedvideo output signal to an output device such as a television screen orvideo recorder to reduce the flickering of the brightness of videoimages from said television camera.
 2. The apparatus of claim 1 andfurther including a table memory, circuitry for delivering saidcorrection factor signal to said table memory, andcircuitry fordelivering the output of said table memory as a multiplication factor tosaid multiplier.
 3. The apparatus of claim 1 or 2 wherein saidtelevision camera includes an interline CCD sensor.